The present invention relates to automatic dispersion equalization apparatus in high-speed optical transmission systems, with bit rates exceeding several tens of Gbit/s.
Recently, various communication service applications, for example, email, Internet, telebanking, teleshopping, videoconference, and other multimedia services have become required. To keep pace with such trends in the communication area, data transmission speed in optical communication has increased substantially. Currently, optical communication systems with 10 Gbit/s speed are widely used, and it is not difficult to expect optical communication systems with 40 Gbit/s or faster to be extensively used in the near future.
However, as the transmission speed of the optical communication systems increases to the range of several tens of Gbit/s, the chromatic dispersion effect of optical fibers becomes one of the most troublesome barriers against efficient high-speed data transmission. In fact, since the allowable amount variation of chromatic dispersion in 40 Gbit/s optical communication systems is within several tens of ps/nm, accurate chromatic dispersion compensation and equalization is required.
An alternative is to build a whole new optical communication systems with optical fibers that can handle chromatic dispersion efficiently. But the expense for this alternative is quite enormous. Linear chromatic dispersion compensation is becoming popular since a pre-installed optical fiber network can be used without additional expense. In linear chromatic dispersion compensation, optical transmitters and receivers are equipped with chromatic dispersion equalizers.
There are three conventional ways to compensate optical fiber chromatic dispersion.
First, linear passive chromatic dispersion method is available. Devices such as dispersion compensating fiber (DCF) and fiber grating implement it
Second, non-linear active chromatic dispersion method is available. devices such as mid-span spectral inversion (MSSI) implement it.
Third, an electric compensation method such as pre-chip, dispersion supported transmission (DST), and duobinary coding which is implemented in transmitters and receivers is available.
In super high-speed optical network systems like systems with 40 Gbit/s, however, it is not easy to equalize chromatic dispersion because the allowable amount of chromatic dispersion is very small, and it is sensitive to environmental factor change like temperature.
Due to reasons stated above, an automatic dispersion equalizer that automatically compensates chromatic dispersion becomes a necessary device in super high-speed optical network systems.
Generally, a chromatic dispersion detector and a chromatic dispersion-variable equalizer are needed in order to implement automatic dispersion equalizer. A detector detects optimal chromatic dispersion value from the physical amount that is repeatedly changed by chromatic dispersion value. An equalizer has a capability of changing its chromatic dispersion value. [xe2x80x9cAutomatic dispersion equalization by monitoring extracted-clock power level in a 40-Gbit/s, 200 km transmission linexe2x80x9d, Sano, Akihide et al., 22nd European Conference on Optical Communication ""96, pp207xcx9c210]
Signals measured by the detector are supplied back to the chromatic dispersion equalizer and automatic dispersion equalization is performed. A well-known technique for detection is to measure error bit ratio, eye opening, and size of clock component of signals transmitted to an optical receiver. Until recently, chromatic dispersion has been measured by this technique and measured data has been provided to external processors for computations. Then, a controller controls the chromatic dispersion generator to equalize chromatic dispersion.
A widely used chromatic dispersion equalizer is an individual linear passive device such as dispersion compensating fiber (DCF), fiber grating, and planar lightwave circuit. Another technique in which feedback signals of a detector are supplied to a transmitter and the output signal frequency of optical source is modified have been introduced.
However, since these techniques require external measurement devices, a processor, and a controller, it is complicated to apply them to high-speed optical communication systems. In addition, from a system perspective, when the system is being used, the chromatic dispersion equalization may degrade performance of system.
The present invention provides an automatic dispersion equalization apparatus in a high-speed optical transmission system, which performs continuous automatic dispersion equalization without the help of external digital processors and performance degradation.
An automatic dispersion equalization apparatus in high-speed optical transmission system is composed of means for generating electric signals in accordance with chromatic dispersions and means for equalizing optical fiber chromatic dispersion by using the electric signals.
More particularly, the automatic dispersion equalization apparatus in a high-speed optical transmission system is composed of the following devices. A chromatic dispersion-variable equalizer adds chromatic dispersion to a positive positive (+) or a negative (xe2x88x92) direction with the help of a positive positive (+) or a negative (xe2x88x92) DC electric signal and thereby equalizes optical fiber chromatic dispersion. An optical coupler receives optical signals from the chromatic dispersion-variable equalizer and splits them into two paths. A chromatic dispersion generator generates chromatic dispersion to the positive (+) or negative (xe2x88x92) direction to add to the optical signals of the optical coupler.
A signal generator generates sine or trigonometrical functions to control chromatic dispersion of the dispersion generator. A photo detector converts the optical signals of the chromatic dispersion generator into electric signals. A narrow band amplifier amplifies electric signals of the photo detector. A narrow bandpass filter extracts clock components of signal speed from the amplified signals. An amplitude detector extracts magnitude variation from clock components extracted by the narrow bandpass filter. A phase shifter adjusts the phase of the output signal of the detector to match with phase of the signal of the signal generator. A multiplier detects the phase difference between the phase of the signals from the phase shifter and the phase of the signals from the signal generator. A low pass filter extracts DC components from output signals of the multiplier. An integrator continuously loads a positive (+) or negative (xe2x88x92) DC value to the chromatic dispersion-variable equalizer as the DC value of the signals from the low pass filter varies between positive (+) and negative (xe2x88x92).